Measuring a process parameter of a semiconductor fabrication process using optical metrology

ABSTRACT

To measure a process parameter of a semiconductor fabrication process, the fabrication process is performed on a first area using a first value of the process parameter. The fabrication process is performed on a second area using a second value of the process parameter. A first measurement of the first area is obtained using an optical metrology tool. A second measurement of the second area is obtained using the optical metrology tool. One or more optical properties of the first area are determined based on the first measurement. One or more optical properties of the second area are determined based on the second measurement. The fabrication process is performed on a third area. A third measurement of the third area is obtained using the optical metrology tool. A third value of the process parameter is determined based on the third measurement and a relationship between the determined optical properties of the first and second areas.

This application is a Continuation of U.S. patent application Ser. No.11/639,515, entitled MEASURING A PROCESS PARAMETER OF A SEMICONDUCTORFABRICATION PROCESS USING OPTICAL METROLOGY, filed on Dec. 15, 2006,which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present application generally relates to optical metrology, and,more particularly, to measuring a process parameter of a semiconductorfabrication process using optical metrology.

2. Related Art

Optical metrology involves directing an incident beam at a structure,measuring the resulting diffracted beam, and analyzing the diffractedbeam to determine a feature of the structure. In semiconductormanufacturing, optical metrology is typically used for qualityassurance. For example, after fabricating a structure on asemi-conductor wafer an optical metrology tool is used to determine theprofile of the structure. By determining the profile of the structure,the quality of the fabrication process utilized to form the structurecan be evaluated.

Optical metrology has been used to evaluate, monitor, or control asemiconductor fabrication process by determining whether the profiles ofthe structures formed by the semiconductor fabrication process arewithin acceptable tolerances. Optical metrology, however, has not beenused to measure a process parameter used in performing the semiconductorfabrication process.

SUMMARY

In one exemplary embodiment, to measure a process parameter of asemiconductor fabrication process, the fabrication process is performedon a first area using a first value of the process parameter. Thefabrication process is performed on a second area using a second valueof the process parameter. A first measurement of the first area isobtained using an optical metrology tool. A second measurement of thesecond area is obtained using the optical metrology tool. One or moreoptical properties of the first area are determined based on the firstmeasurement. One or more optical properties of the second area aredetermined based on the second measurement. The fabrication process isperformed on a third area. A third measurement of the third area isobtained using the optical metrology tool. A third value of the processparameter is determined based on the third measurement and arelationship between the determined optical properties of the first andsecond areas.

DESCRIPTION OF DRAWING FIGURES

The present application can be best understood by reference to thefollowing description taken in conjunction with the accompanying drawingfigures, in which like parts may be referred to by like numerals:

FIG. 1 depicts an exemplary fabrication tool and an exemplary opticalmetrology tool;

FIG. 2 depicts in more detail the exemplary optical metrology tooldepicted in FIG. 1; and

FIG. 3 depicts an exemplary process of measuring a process parameterused to perform a semiconductor fabrication process in the exemplaryfabrication tool using the exemplary optical metrology tool depicted inFIG. 1.

DETAILED DESCRIPTION

The following description sets forth numerous specific configurations,parameters, and the like. It should be recognized, however, that suchdescription is not intended as a limitation on the scope of the presentinvention, but is instead provided as a description of exemplaryembodiments.

FIG. 1 depicts one or more wafers 102 processed in an exemplaryfabrication tool 104. One or more semiconductor fabrication processescan be performed on one or more wafers 102 in fabrication tool 104.Typically a number of wafers 102 are processed as a batch, commonlyreferred to as a wafer lot, in fabrication tool 104. For example, awafer lot of 25 wafers 102 can be processed as a batch in fabricationtool 104. It should be recognized, however, that the number of wafer 102in a wafer lot can vary.

One or more process parameters are used in performing the one or moresemiconductor fabrication processes. Typically, the one or more processparameters are set to define a recipe. Also, the same recipe (i.e., asetting of the one or more process parameters) is used to process thewafers in one wafer lot. One or more individual process parameters of aparticular recipe can be adjusted while processing the wafers in onewafer lot. The one or more process parameters can also be set todifferent values to define different recipes. Different recipes can beused to process different wafer lots. Thus, one recipe can be used toprocess one wafer lot, and another recipe can be used to process anotherwafer lot.

For example, fabrication tool 104 can be a coater/developer tool, whichis used to deposit and develop a photoresist layer on one or more wafers102. The one or more process parameters used to perform the depositionand development processes can include temperature, spin speed, spintime, and the like. Thus, in this example, any one or more of theseprocess parameters can be set to define any number of different recipes.The variation of a process parameter can produce a variation in thephotoresist layer deposited and/or developed using fabrication tool 104.For example, different spin speeds can change the thickness and/oruniformity of the photoresist layer deposited on one or more wafers 102.It should be recognized that fabrication tool 104 can be various typesof fabrication tools, such as a plasma etch tools, cleaning tools,chemical vapor deposition (CVD) tools, and the like.

As depicted in FIG. 1, after one or more semiconductor fabricationprocesses are performed on one or more wafers 102 in fabrication tool104, one or more wafers 102 can be examined using optical metrology tool106. As will be described in more detail below, optical metrology tool106 can be used to determine one or more features of a structure formedon one or more wafers 102. Optical metrology tool 106 can also be usedto measure a process parameter used in performing the semiconductorfabrication process on one or more wafers 102 in fabrication tool 104.

As depicted in FIG. 2, optical metrology tool 106 can include aphotometric device with a source 204 and a detector 206. A structure 202formed on wafer 102 is illuminated by an incident beam from source 204.Diffracted beams are received by detector 206. Detector 206 converts thediffracted beam into a measured diffraction signal, which can includereflectance, tan(Ψ), cos(Δ), Fourier coefficients, and the like.Although a zero-order diffraction signal is depicted in FIG. 2, itshould be recognized that non-zero orders can also be used.

Optical metrology tool 106 also includes a processing module 208configured to receive the measured diffraction signal and analyze themeasured diffraction signal. Processing module 208 can include aprocessor 210 and a computer-readable medium 212. It should berecognized, however, that processing module 208 can include any numberof components in various configurations.

In one exemplary embodiment, processing module 208 is configured todetermine one or more features of structure 202 using any number ofmethods which provide a best matching diffraction signal to the measureddiffraction signal. These methods can include a library-based process,or a regression based process using simulated diffraction signalsobtained by rigorous coupled wave analysis and machine learning systems.See, U.S. Pat. No. 6,943,900, titled GENERATION OF A LIBRARY OF PERIODICGRATING DIFFRACTION SIGNALS, filed on Jul. 16, 2001, issued Sep. 13,2005, which is incorporated herein by reference in its entirety; U.S.Pat. No. 6,785,638, titled METHOD AND SYSTEM OF DYNAMIC LEARNING THROUGHA REGRESSION-BASED LIBRARY GENERATION PROCESS, filed on Aug. 6, 2001,issued Aug. 31, 2004, which is incorporated herein by reference in itsentirety; U.S. Pat. No. 6,891,626, titled CACHING OF INTRA-LAYERCALCULATIONS FOR RAPID RIGOROUS COUPLED-WAVE ANALYSES, filed on Jan. 25,2001, issued May 10, 2005, which is incorporated herein by reference inits entirety; and U.S. patent application Ser. No. 10/608,300, titledOPTICAL METROLOGY OF STRUCTURES FORMED ON SEMICONDUCTOR WAFERS USINGMACHINE LEARNING SYSTEMS, filed on Jun. 27, 2003, which is incorporatedherein by reference in its entirety.

In one exemplary embodiment, processing module 208 is configured tomeasure a process parameter used in performing the semiconductorfabrication process on one or more wafers 102 in fabrication tool 104(FIG. 1). In particular, FIG. 3 depicts an exemplary process 300 tomeasure a process parameter of a semiconductor fabrication process.

In step 302, the semiconductor fabrication process is performed on afirst area using a first value of the process parameter. For example, asdescribed above, the semiconductor fabrication process can be a coaterprocess in which a layer of photoresist is deposited on the wafer.Assume that the coater process involves spinning the wafer, and that theprocess parameter of interest is the spin speed. Thus, in the presentexample, assume that a photoresist layer is deposited on the first areausing a first spin speed.

In step 304, the semiconductor fabrication process is performed on asecond area using a second value of the process parameter. Returning tothe example above in which the semiconductor fabrication process is acoater process and the process parameter is spin speed, assume that aphotoresist layer is deposited on the second area using a second spinspeed.

Note that in a typical coater process the layer of photoresist isdeposited across the entire wafer. Thus, in the example above, the firstarea can correspond to a location on one wafer and the second area cancorrespond to a location on another wafer. It should be recognized,however, that some semiconductor fabrication processes can be performedspecific to particular locations on a single wafer. Thus, the first andsecond areas can correspond to different locations on a single wafer.Also, the first and second areas can correspond to locations ondifferent wafers in different wafer lots or the same wafer lot.

In step 306, a first measurement of the first area is obtained using anoptical metrology tool. As described above, with reference to FIG. 2, anexemplary optical metrology tool 106 can include a photometric devicewith a source 204 and a detector 206. The first area can be illuminatedby an incident beam from source 204. Diffracted beams from the firstarea are received by detector 206. Detector 206 converts the diffractedbeam into a first measured diffraction signal, which can includereflectance, tan(Ψ), cos(Δ), Fourier coefficients, and the like.

With reference again to FIG. 3, in step 308, a second measurement of thesecond area is obtained using the optical metrology tool. With referenceagain to FIG. 2, the second area can be illuminated by an incident beamfrom source 204. Diffracted beams from the second area are received bydetector 206. Detector 206 converts the diffracted beam into a secondmeasured diffraction signal, which can include reflectance, tan(Ψ),cos(Δ), Fourier coefficients, and the like.

As noted above, the first and second areas can be different locations ona single wafer, locations on different wafers, and the different waferscan be in different wafer lots or the same wafer lot. In one exemplaryembodiment, the first and second areas are thin film areas, which areun-patterned areas.

In step 310, one or more optical properties of the first area aredetermined based on the first measurement. As described above, withreference to FIG. 2, optical metrology tool 106 also includes aprocessing module 208 configured to receive the measured diffractionsignal and analyze the measured diffraction signal. As also describedabove, processing module 208 can be configured to determine one or morefeatures of structure 202 using any number of methods which provide abest matching diffraction signal to the measured diffraction signal. Inthe present embodiment, the one or more optical properties of the firstarea can be the one or more features determined by processing module 208using the first measured diffraction signal. Additionally, as notedabove, in one exemplary embodiment, the first area being examined is athin film area rather than having structure 202.

With reference again to FIG. 3, in step 312, one or more opticalproperties of the second area are determined based on the secondmeasurement. With reference again to FIG. 2, similar to the one or moreoptical properties of the first area, the one or more optical propertiesof the second area can be the one or more features determined byprocessing module 208 using the second measured diffraction signal.Additionally, as noted above, in one exemplary embodiment, the secondarea being examined is a thin film area rather than having structure202.

With reference again to FIG. 3, in step 314, the semiconductorfabrication process is performed on a third area. Returning to theexample above in which the semiconductor fabrication process is a coaterprocess and the process parameter is spin speed, assume that aphotoresist layer is deposited on the third area. Note, the third areacan be a different location than the first and second areas on a singlewafer, a location on a different wafer than the first and second area,and the different wafers can be in different wafer lots or the samewafer lot.

In step 316, a third measurement of the third area is obtained using theoptical metrology tool. With reference again to FIG. 2, the third areacan be illuminated by an incident beam from source 204. Diffracted beamsfrom the third area are received by detector 206. Detector 206 convertsthe diffracted beam into a third measured diffraction signal, which caninclude reflectance, tan(Ψ), cos(Δ), Fourier coefficients, and the like.

Note again that the third area can be a different location than thefirst and second areas on a single wafer, a location on a differentwafer than the first and second area, and the different wafers can be indifferent wafer lots or the same wafer lot. In one exemplary embodiment,the third area is also a thin film area.

In step 318, a third value of the process parameter is determined basedon the third measurement and a relationship between the determinedoptical properties of the first and second areas. In one exemplaryembodiment, a range of potential values for the process parameter ofinterest is determined. The first value corresponds to the minimum valuein the range. The second value corresponds to the maximum value in therange. The third value is within the range of potential values.

In one exemplary embodiment, the third value of the process parameter isdetermined using a mixing coefficient from the relationship between thedetermined optical properties of the first and second areas. The thirdvalue of the process parameter is determined using the determined mixingcoefficient and a relationship between the first value and the secondvalue of the process parameter.

For example, assume that the one or more optical properties of the firstarea include refractive index (n) and extinction coefficient (k) of thefirst area, which are commonly referred to as n & k. Assume also thatthe one or more optical properties of the second area include n & k ofthe second area, and the one or more optical properties of the thirdarea include n & k of the third area. Thus, in this example, the n & kof the third area is determined using a relationship between the n & kdetermined of the first and second areas. In one exemplary embodiment,the n & k of the third area is determined using a mixing coefficientfrom the relationship between the determined n & k of the first andsecond areas. The third value of the process parameter is determinedusing the mixing coefficient and a relationship between the first valueand the second value of the process parameter.

As a further example, assume that the relationship between thedetermined n & k of the first and second areas is expressed using aneffective medium approximation (EMA) formula as follows:

$\begin{matrix}{\frac{ɛ - ɛ_{h}}{ɛ + {\alpha ɛ}_{h}} = {\sum\limits_{i = 1}^{m}{\upsilon_{i}{\frac{ɛ_{i} - ɛ_{h}}{ɛ_{i} + {\alpha ɛ}_{h}}.}}}} & (1)\end{matrix}$In equation (1), υ_(i), is a mixing coefficient, which can vary from 0to 1. m is the number of areas measured. In the example above, m=2. Itshould be recognized, however, that any number of areas can be measured.∈=(n+j*k)², which is a complex function of n & k (j=sqrt(−1)), and iscalled a dielectric constant.

Either or both sides of equation (1) can be simplified based on certainassumptions. For example, assuming a spherical microstructure, α can beassumed to be equal to 2. Also, one of the following can be assumed aswell:

-   -   1) Linear assumption, where ∈=1;    -   2) Maxwell-Garnet assumption, where ∈_(h)=∈_(i); or    -   3) Bruggeman assumption, ∈_(h)=∈.        Thus, equation (1) can be solved for the mixing coefficient,        υ_(i).

The optical properties used to solve equation (1) correspond to valuesof the process parameter of interest. In the example above, the opticalproperties correspond to two values of spin speed used in a depositionprocess. In particular, assume the first value of the process parameteris S₁, and the second value of the process parameter is S₂.

The mixing coefficient can also be used in defining a relationshipbetween the values of the first and second process parameters. As anexample, a linear relationship can be assumed, and the process parameterS can be defined using the relationship:S=S ₁+υ(S ₂ −S ₁).  (2)It should be recognized, however, that the relationship between thevalues of the process parameters can be defined using various functions,including non-linear functions.

After solving for the mixing coefficient, υ, in equation (1), the thirdvalue of the process parameter S can be solved using equation (2) byusing the mixing coefficient, υ, the first value of the processparameter, S₁, and the second value of the process parameter, S₁.

Alternatively, for two measured areas, equation (2) can be solved forthe mixing coefficient, υ, and substituted into equation (1). Equation(1), which now has the relationship between the values of the processparameters integrated into it, can then be solved for the processparameter of interest, S. Thus, the process parameter of interest, S,can be determined directly from the third measurement and therelationship between the determined optical properties integrated withthe relationship between the values of the process parameters.

Although in the examples described above the relationship used todetermine the value of the process parameter is based on the determinedoptical properties of two areas (the first and second areas), it shouldbe recognized that the relationship can be determined based on opticalproperties of more than two areas. For example, the semiconductorfabrication process can be performed on three or more areas,measurements can be obtained from the three or more areas, and opticalproperties can be determined of the three of the more areas based on themeasurements obtained from the three or more areas.

For more than two measured areas, equation (2) can be more generallyexpressed as follows:

$\begin{matrix}{S = {\sum\limits_{i = 1}^{m}{\upsilon_{i}{S_{i}.}}}} & (3)\end{matrix}$

Also, the relationship between n & k and the process parameter can beexpressed more generally using the following function:∈=a ₀ +a ₁ S+a ₂ S+ . . . +a _(m−1) S ^(m−1),  (4)where m is the number of measured areas.

After the third value of the process parameter is determined, the thirdvalue can be used to adjust the semiconductor fabrication processperformed in the fabrication tool. For example, with reference to FIG.1, after determining the third value using optical metrology tool 106,the third value can be used to adjust the process parameter of thesemiconductor fabrication process performed in fabrication tool 104.

In particular, as described above, in addition to determining the thirdvalue, optical metrology tool 106 can determine one or more features ofa structure formed on one or more wafer 102. Thus, when the third valueis determined for a particular wafer, one or more features of astructure formed on that particular feature can also be determined. Ifthe one or more features are not within acceptable tolerances, then theprocess parameter of the semiconductor fabrication process performed infabrication tool 104 can be adjusted based on the determined thirdvalue.

As a specific example, assume again that the semiconductor fabricationprocess is a deposition process to deposit a layer of photoresist andthat the process parameter of interest is spin speed. After a layer ofphotoresist has been deposited on a wafer using fabrication tool 104,the thickness of the layer of photoresist can be determined as one ofthe features determined using optical metrology tool 106. A spin speedcan also be determined for the wafer using optical metrology tool 106.If the determined thickness is not within acceptable tolerances (e.g.,too thick or thin), then the spin speed used in the deposition processin fabrication tool 104 can be adjusted based on the determined spinspeed.

Although fabrication tool 104 and optical metrology tool 106 aredepicted in FIG. 1 as separate tools, it should be recognized thatoptical metrology tool 106 can be integrated into fabrication tool 104.For example, with reference to FIG. 2, source 204 and detector 206 ofoptical metrology tool 106 can be mounted on fabrication tool 104(FIG. 1) to examine wafers 102 after they are processed in fabricationtool 104 (FIG. 1).

It should be recognized that the steps of process 300 (FIG. 3) can beperformed by processor 210 based on computer-readable instructionsstored on computer-readable medium 212. It should also be recognized,however, that the steps of process 300 (FIG. 3) can be performed usingvarious combinations and configurations of hardware and/or software.

Although exemplary embodiments have been described, variousmodifications can be made without departing from the spirit and/or scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the specific forms shown in the drawingsand described above.

1. A method of measuring a process parameter of a semiconductorfabrication process, the method comprising: obtaining a firstdiffraction signal measured from a first area using an optical metrologytool, wherein the semiconductor fabrication process was performed on thefirst area using a first value of the process parameter; obtaining asecond diffraction signal measured from a second area using the opticalmetrology tool, wherein the semiconductor fabrication process wasperformed on the second area using a second value of the processparameter that is different than the first value; determining one ormore optical properties of the first area based on the first diffractionsignal; determining one or more optical properties of the second areabased on the second diffraction signal; obtaining a third diffractionsignal measured from a third area using the optical metrology tool,wherein the semiconductor fabrication process was performed on the thirdarea, and wherein the first, second, and third areas are different areason the same wafer or different wafers; and determining a third value ofthe process parameter used to perform the semiconductor fabricationprocess on the third area based on the third diffraction signal and arelationship between the determined optical properties of the first andsecond areas.
 2. The method of claim 1, wherein determining a thirdvalue comprises: determining a mixing coefficient from the thirddiffraction signal and the relationship between the determined opticalproperties of the first and second areas; and determining the thirdvalue of the process parameter using the determined mixing coefficientand a relationship between the first value and the second value of theprocess parameter.
 3. The method of claim 2, wherein the mixingcoefficient is determined based on an interpolation between the opticalproperties of the first and second areas.
 4. The method of claim 1,wherein a relationship between the first value and the second value ofthe process parameter is integrated into the relationship between thedetermined optical properties of the first and second areas, and whereinthe third value of the process parameter is determined from the thirddiffraction signal and the relationship between the determined opticalproperties of the first and second areas integrated with therelationship between the first value and the second value of the processparameter.
 5. The method of claim 1, wherein the optical properties ofthe first area includes refractive index and extinction coefficient ofthe first area, and the optical properties of the second area includesrefractive index and extinction coefficient of the second area.
 6. Themethod of claim 1, wherein the first, second, and third diffractionsignals are obtained from thin film areas.
 7. The method of claim 1,wherein the one or more optical properties of the first and second areasare determined by comparing the diffraction signals measured from thefirst and second areas to one or more simulated diffraction signals. 8.The method of claim 1, wherein the different wafers are in differentwafer lots.
 9. The method of claim 1, wherein the different wafers arein the same wafer lot.
 10. The method of claim 1, further comprising:defining a range of potential values for the process parameter, therange having a minimum value and a maximum value, wherein the firstvalue conesponds to the minimum value and the second value correspondsto the maximum value, and wherein the third value is within the range ofpotential values.
 11. The method of claim 1, further comprising:adjusting the semiconductor fabrication process based on the determinedthird value.
 12. A computer-readable storage medium having computerexecutable instructions to measure a process parameter of asemiconductor fabrication process, comprising instructions for:obtaining a first diffraction signal measured from a first area using anoptical metrology tool, wherein a first value of the process parameterwas used in performing the semiconductor fabrication process on thefirst area; obtaining a second diffraction signal measured from a secondarea using the optical metrology tool, wherein a second value of theprocess parameter was used in performing the semiconductor fabricationprocess on the second area; determining one or more optical propertiesof the first area based on the first diffraction signal; determining oneor more optical properties of the second area based on the seconddiffraction signal; obtaining a third diffraction signal measured from athird area using the optical metrology tool; and determining a thirdvalue of the process parameter used to perform the semiconductorfabrication process on the third area based on the third diffractionsignal and a relationship between the determined optical properties ofthe first and second areas, wherein the first, second, and third areasare different areas on the same wafer or different wafers.
 13. Thecomputer-readable storage medium of claim 12, wherein determining athird value comprises instructions for: determining a mixing coefficientfrom the third measurement and the relationship between the determinedoptical properties of the first and second areas; and determining thethird value of the process parameter using the determined mixingcoefficient and a relationship between the first value and the secondvalue of the process parameter.
 14. The computer-readable storage mediumof claim 12, wherein a relationship between the first value and thesecond value of the process parameter is integrated into therelationship between the determined optical properties of the first andsecond areas, and wherein the third value of the process parameter isdetermined from the third measurement and the relationship between thedetermined optical properties of the first and second areas integratedwith the relationship between the first value and the second value ofthe process parameter.
 15. The computer-readable storage medium of claim12, wherein the optical properties of the first area includes refractiveindex and extinction coefficient of the first area, and the opticalproperties of the second area includes refractive index and extinctioncoefficient of the second area, and wherein the first and secondmeasurements are diffraction signals measured using the opticalmetrology tool.
 16. The computer-readable storage medium of claim 12,further comprising instructions for: defining a range of potentialvalues for the process parameter, the range having a minimum value and amaximum value, wherein the first value corresponds to the minimum valueand the second value conesponds to the maximum value, and wherein thethird value is within the range of potential values.
 17. Thecomputer-readable storage medium of claim 12, further comprisinginstructions for: adjusting the semiconductor fabrication process basedon the determined third value.
 18. A system to measure a processparameter of a semiconductor fabrication process, the system comprising:a fabrication tool configured to perform the semiconductor fabricationprocess on a first area using a first value of the process parameter, asecond area using a second value of the process parameter, and a thirdarea, wherein the first, second, and third areas are different areas onthe same wafer or different wafers; and an optical metrology toolconfigured to: measure a first diffraction signal from the first area;measure a second diffraction signal from the second area; determine oneor more optical properties of the first area based on the firstdiffraction signal; determine one or more optical properties of thesecond area based on the second diffraction signal; measure a thirddiffraction signal from the third area; and determine a third value ofthe process parameter used to perform the semiconductor fabricationprocess on the third area based on the third diffraction signal and arelationship between the determined optical properties of the first andsecond areas.
 19. The system of claim 18, wherein the optical propertiesof the first area includes refractive index and extinction coefficientof the first area, and the optical properties of the second areaincludes refractive index and extinction coefficient of the second area.20. The system of claim 18, wherein the different wafers are indifferent wafer lots.
 21. The system of claim 18, wherein the differentwafers are in the same wafer lot.
 22. The system of claim 18, whereinthe optical metrology tool is configured to adjust the semiconductorfabrication process based on the determined third value.